1. Field of the Invention
The present invention relates generally to a motor control device for controlling the rotation of a motor, and more particularly, to an improvement in braking control in a DC motor.
Furthermore, the present invention relates to a motor control device suitable for an image forming apparatus having a paper holder for holding paper to which an image is to be transferred such as a transfer drum.
2. Description of the Prior Art
Various types of motor control devices for rotating a DC motor at a constant speed have been conventionally proposed For example, U.S. Pat. No. 4,623,827 discloses a motor control device for detecting the speed of a DC motor by a pulse width of a rotary encoder mounted on the motor and carrying out feedback control of the motor on the basis of the difference between the detected speed and a predetermined set speed.
In this type of conventional motor control device, the difference between a command speed and the detected speed is generally found, and the driving of the motor is controlled by a control signal proportional to the difference in speed. When the rotational speed of the motor is decreased from a predetermined high speed to a predetermined low speed, control is carried out using the following procedure. More specifically, the command speed is switched from the predetermined high speed to the predetermined low speed. Consequently, there arises a large difference between the command speed and the detected speed Accordingly, a control signal proportional to this large difference in speed is applied to the motor, so that the speed of the motor which is being rotated at a high speed is decreased to the command speed.
However, this control has the disadvantage in that it takes long time for the rotational speed of the motor to reach the predetermined low speed because the speed of the motor is decreased, not by forced braking, but only by the control signal.
Furthermore, as the prior art of interest to the present invention, a color copying machine having a transfer drum has been known (see, for example, Japanese Patent Laid-Open Gazette No. 113481/1988). In this color copying machine, such a mechanism is used that at the time of copying, paper is held with it being wound around the peripheral surface of the transfer drum, and the transfer drum around which the paper is wound is rotated at a process speed for image formation with it being abutted on a photosensitive drum, to transfer to the paper an image formed on the photosensitive drum.
When the paper is fed to the transfer drum, the paper delivered is stopped once at the point in time when the front end thereof reaches a registration roller provided short of the transfer drum The paper is fed to the transfer drum at predetermined timing by the registration roller. That is, so-called secondary paper feeding is achieved.
In this type of copying machine, the speed at which paper is conveyed is made higher than the peripheral speed of the transfer drum, that is, the process speed when the secondary paper feeding is started, and is returned to the process speed when the front end of the paper reaches the transfer drum. Such a change in speed has been conventionally made by a speed changing clutch connected to the registration roller.
The speed at which paper is conveyed is thus switched in the secondary paper feeding for the following reasons. More specifically, the variation in rise time of a registration roller driving clutch, the variation in distance from the registration roller to the transfer drum due to mounting errors of components of the copying machine, the distance between the position of the registration roller itself and a switch for detecting that the front end of the paper reaches the registration roller, and the like must be absorbed
In such construction that the speed at which paper is conveyed is switched by the speed changing clutch, however, there is no problem while the copying machine is new, but a problem is encountered that the speed changing clutch is slid due to changes with age.
Furthermore, in a method of switching the speed using the speed changing clutch, there is a variation in time for switching by the clutch. Accordingly, there is a problem that it takes a lot of time and labor to make adjustments because copying machines must be adjusted one at a time with a clutch being mounted thereon In addition, there is a problem that only unstable adjustments can be made because it is difficult to judge with eyes whether or not the timing of operating the clutch is suitable
The problems should be avoided by not switching the rotational speed of the registration roller using the speed changing clutch, but by providing a dedicated motor and changing the rotational speed itself of the motor to switch the rotational speed of the registration roller.
The present invention has been made on the basis of such a technical background.
Meanwhile, various types of control devices for controlling the rotational speed of a motor at a constant speed have been proposed as described above.
However, the use of the conventionally known motor control device without any modification is not suitable for control of the registration roller driving motor in the above described copying machine. The reason for this is that the registration roller driving motor must be rotated at a high speed when the secondary paper feeding is started and must be so controlled that the speed thereof is decreased to a process speed in a short time and without any variation after an elapse of a constant time.